Many synthetic polymers have characteristics that make them useful as biomedical materials. One reason for this is the wide range of properties available from man-made polymers, The chemistry of the repeat unit, the shape of the molecular backbone, and the existence and concentration of intermolecular bonds among the millions of molecules that make up the polymer sample all influence ultimate properties. Additional property variations are possible in polymers with more than one kind of repeating unit. Copolymers, terpolymers, and even multipolymers are possible in which the properties of more than one polymer type are combined to produce a unique material. The arrangement of the different repeat units in copolymers allows further property variations. The overall concentration of each monomer is also a major determinant of the properties of copolymers, but unless one monomer is used in great excess over the other, the resulting properties can be quite different from either homopolymer.
In graft and block copolymers, particularly when graft or block length is high, some of the properties of the two homopolymers are retained. For instance, a hard, high-melting block can by copolymerized with a soft rubbery block. With the proper arrangement of the blocks, the resulting copolymer can be a thermoplastic elastomer. At room temperature, the liquid-like soft blocks are strengthened and reinforced by the hard blocks or segments. At elevated temperatures, the hard blocks soften and flow to permit thermoplastic processing. Upon cooling, the original structure re-forms. The thermoplastic polyurethanes, which is an important class of biomaterials, have this block, or segmented, structure. Many interesting polymers can be made by combining one hard block with two or three different soft blocks. These polymers can have interesting permeability properties and biocompatibility, both of which can be tailored over a wide range by varying block chemistry and concentration.
In addition to the structural factors mentioned, the shape of a polymer's molecular weight distribution and its average molecular weight can have a significant effect on polymer properties. If one were to fractionate a typical polymer sample according to chain length, one might find that the low molecular weight homologues were waxes or even liquids, while the high molecular weight fractions were tough and viscous, even at elevated temperatures. The macroscopic properties that are measured and assigned to polymers are really the weighted averages of the properties of the various polymer fractions that are present in the sample.
Although the life-threatening consequences of inadequate biocompatibility in an artificial heart are well appreciated, lack of biocompatibility is seldom implicated when complications occur with simple acute devices such as vascular catheters. In fact, all blood and tissue contacting devices could probably benefit from improved biomaterials. Clotting, inflammatory response, and infection in even the simplest devices can result in sudden death of irreversible damage to the patient. The blood-materials interactions that occur at a smooth surface are affected only by the constitution of the outer few molecular monolayers of the polymer. This means that as long as the polymer does not contain any leachable impurities, the chemistry of the bulk polymer, which is distant from the biological interface, does not affect in vivo performance.
Many commercially available polymers contain additives or impurities that are surface-active. A surface-active agent, or surfactant, is capable of migrating to an interface and populating that interface at a concentration that is much higher than its average concentration in the bulk phase. Extremely surface-active materials can have nearly 100 percent concentration in a surface, even if their initial bulk or average concentration in the polymer is in the parts per million range. This is analogous to the effect a detergent has on the surface tension and surface chemistry of water. Accordingly, trying to interpret the surface analysis of a polymer contaminated with unknown substance is very difficult. In the absence of sensitive surface analysis, a sample's biological response may be wrongly assigned to the base polymer when it is, in fact, largely due to a contaminant. Processing or thermal history variations can lead to variability in in vivo performance if differences in the amounts of additive or impurity in the surface are produced.
Certain block and graft copolymers can add additional complexity to the relationship between surface chemistry and bulk chemistry. Solids and liquids try to minimize interfacial energy. This is the same driving force that causes low energy surface-active impurities to migrate to the air-facing surface of a polymer. Since air is a low-energy fluid, the interface between air and the polymer will have the lowest energy when the polymer surface also has a low energy. Migration of the surfactant to the polymer surface succeeds in lowering polymer surface energy and, therefore, overall interfacial energy. This effect is thought to minimize the activation of blood constituents for coagulation, cell adhesion, and other adverse biological processes.
In many block and graft copolymers, another mechanism for interfacial energy minimization exists. By reorientation of the surface molecular layers, one of the blocks or grafts can preferentially populate the surface. For instance, when brought to equilibrium in air, a block copolymer comprised of high surface energy hard segments and low surface energy soft segments will have a surface that is mostly comprised of the so-called soft block or low surface energy block. It is even possible that none of the more polar, hard segment will be present in the polymer surface. A polymer put into the blood stream is exposed to the more polar, aqueous environment of the blood. The polymer may then attempt to reorient its polar blocks toward the surface in order to minimize the energy of the blood-polymer interface.